FIG. 1 of the accompanying drawings illustrates a typical known type of cable tuner of the double conversion type. The tuner has an antennae input connected to the input of an automatic gain control (AGC) stage 1 for controlling the signal level supplied to a first frequency changer 2 so as to maximise the signal to intermodulation plus noise performance of the tuner. The AGC stage 1 typically comprises a PIN diode whose input resistance can be varied by varying a standing current therethrough. The resulting variable resistance is used as part of a potential divider for attenuating the input signal in accordance with a gain control signal, which may be derived from a measurement of the signal level downstream of the stage 1 or from a demodulator (not shown).
The first frequency changer 2 comprises a mixer 2a which receives a local oscillator signal from a local oscillator (LO) 2b controlled by a phase locked loop (PLL) synthesiser 3. The first frequency changer 2 performs block up-conversion of all of the channels supplied to the tuner input, for example with the frequency of the desired channel being centred on the first high intermediate frequency, typically 1.1 GHz.
The output of the first frequency changer 2 is supplied to a first intermediate frequency filter 4, which has a defined centre frequency and passband characteristic. Typically, the filter 4 is a bandpass filter whose centre frequency is nominally equal to the first intermediate frequency and whose passband is such that the filter passes a small number of individual channels.
The output of the filter 4 is supplied to a second frequency changer 5, which similarly comprises a mixer 5a and a local oscillator 5b controlled by a phase locked loop synthesiser 6. The second frequency changer 5 performs a block down conversion such that the desired channel is centred on the second intermediate frequency, for example 36 MHz. The output of the second frequency changer 5 is supplied to a second intermediate frequency filter 7, which has a passband substantially equal to a single channel bandwidth and, for example, a shaped passband characteristic as defined by the modulation standard of the received signal. The filter 7 passes the desired channel and substantially eliminates all other channels. The filtered signal is supplied to an amplifier 8 and to the “IF output” of the tuner, which is connected to a suitable demodulator (not shown) which may be of analog or digital type.
The tuner shown in FIG. 1 may be connected to a cable distribution network for receiving television signals, data signals, telephony signals, or combinations thereof. The tuner thus functions as a “port” to the distribution network and is required to meet minimum requirements so as to prevent or reduce interference or similar effects to other users connected to the network while generating an acceptable level of distortion for the received signal. For example, the input of the tuner is required to have a defined input impedance with a minimum return loss so that energy reflected back into the distribution network is at an acceptably low level. Also, the tuner is required to inject an acceptably low level of potentially interfering signals back into the distribution network. Such signals generated within the tuner and potentially injected into the distribution network include local oscillator re-radiation, local oscillator/local oscillator beats, and spurious products generated by the received signals beating with each other or with the local oscillators.
The tuner is required to generate an acceptably low level of distortion for the received signal so that this can be demodulated to provide acceptable reception for a user. There are various mechanisms within the tuner which degrade the received signal. A first of these is caused by thermal noise which results in a broadband noise spectrum across the received channel. A second of these results from intermodulation caused by beating together of received channels. The input signal is generally of a broadband type comprising a large number of channels and this potentially gives rise to a large number of intermodulation products.
These requirements mean that the part of the tuner near its input in the signal path has to minimise noise and intermodulation generation while presenting a well-controlled characteristic input impedance to the distribution network. In particular, this part of the tuner has to provide a low noise figure (NF), high second and third intermodulation intercepts (IIP2, IIP3), and a relatively frequency-independent input impedance which is generally 75 ohms. Also, the amplitude of signals from the distribution network can vary typically from −5 to +15 dBmV so that this part of the tuner must provide acceptable performance over such a dynamic range of input signal levels. This results in an additional requirement of achieving high intermodulation intercept performance at relatively high input signals, which may be expressed in parametric terms as a relatively high “1 dB compression (P1 dB)”.
These performance requirements are, at least to some extent, mutually exclusive, in particular where the first active stage of the tuner is a mixer as in the tuner of FIG. 1. For example, the mixing function degrades the noise figure by a minimum of 4 dB and the action of the mixer may introduce phasing imbalances in the signal which can result in degraded intermodulation performance.
GB 2 117 588 discloses a dual standard (VHF/UHF) receiver of the double conversion type. Between the signal first mixer and individual inputs for different bands or types of signals, there are three filtering paths, each containing a filter which is tunable so as to perform some pre-selection of the channel selected for reception. Although there are fixed filters in the form of an IF trap in one filter path and a high pass filter in the lower filter path, these do not function in any way to divide the input bandwidth into a plurality of fixed sub-bands.
EP 0 784 381 discloses what appears to be a radio frequency pager which can be switched to operate at two or more fixed frequencies. Although there are two filter paths alternately switchable into circuit between the common input and the common first mixer, each filter path comprises a bandpass filter whose purpose is to select what is effectively a single channel.
U.S. Pat. No. 5,493,717 discloses an FM car radio having an essentially conventional front end. Following the frequency changer, the IF path contains a switched filtering arrangement in which filters having different bandwidths are switched into the IF path according to “signal evaluation circuits” which respond to the level of the IF signal after the first fixed IF filter. The bandwidths of the filters are for selecting a single channel following conversion to IF with different selectivities according to interference conditions.